1. Field of the Invention
The present invention relates in general to memory circuits and, more particularly, to dynamic random access memory cells and a method for forming the same.
2. State of the Art
Random access memory (“RAM”) cell densities have increased dramatically with each generation of new designs and have served as one of the principal technology drivers for ultra large scale integration (“ULSI”) in integrated circuit (“IC”) manufacturing. However, in order to accommodate continuing consumer demand for integrated circuits that perform the same or additional functions and yet have a reduced size as compared with available circuits, circuit designers continually search for ways to reduce the physical size of the memory arrays within these circuits without sacrificing array performance.
With respect to memory ICs, the area required for each memory cell in a memory array partially determines the capacity of a memory IC. This area is a function of the number of elements in each memory cell and the size of each of the elements. For example, FIG. 1 illustrates an array 100 of memory cells 110 for a conventional dynamic random access memory (DRAM) device. Memory cells 110 are typically formed in adjacent pairs, where each pair is formed in a common active area 120 and shares a common source/drain region that is connected to a respective digit line via a digit line contact 124. The area of the memory cells 110 is said to be 8F2, where F represents a minimum feature size for photolithographically defined features. For conventional 8F2 memory cells, the dimension of the cell area is 2F×4F. The dimensions of a conventional 8F2 memory cell are measured along a first axis from the center of a shared digit line contact 124 (½F), across a word line 128 that represents an access transistor (1F), a storage capacitor 132 (1F), an adjacent word line 136 (1F), and half of an isolation region 140 (½F) separating the active area 120 of an adjacent pair of memory cells (i.e., resulting in a total of 4F). The dimensions along a second perpendicular axis are half of an isolation region 150 on one side of the active area 120 (½F), the digit line contact 124 (1F), and half of another isolation region 154 on the other side of the active area 120 (½F) (i.e., resulting in a total of 2F).
In some state-of-the-art memory devices, the memory cells for megabit DRAM have cell areas approaching 6F2. Although this is approximately a 25% improvement in memory cell area relative to conventional 8F2 memory cells, as previously described, a further reduction in memory cell size is still desirable. Therefore, there is a need for a still more compact memory cell structure and method for forming the same.